Polluted water can be purified by using such natural substances as sand or pebbles or by using an artificial filter comprising charcoal, filter paper, sponge or the like. In recent times we also use bacteria for that purpose. That is, we can encourage the growth of bacteria in a treating tank or introduce them from outside into the tank in order for them to attack the organic material in the polluted water and decompose it quickly. Of the various bacteria, Thiobacillus ferrooxidans is very unique. This bacteria oxidize ferrous iron ions in the water to ferric iron ions. The bacteria are called "chemoautotrophic bacteria" in microbiology. In addition to the above function, it oxidizes sulfur to sulfuric acid. This is a geomicrobiologically important function.
On the other hand, in the modern world the waste water discharged from factories, mines, refineries, and other industrial places, as well as from households, reaches a vast quantity. So sewage treatment is a very serious social problem in most need of attention. In particular, the waste water from mines contains, among others, such metals as iron, copper, and zinc. Then, it is economically important whether the iron contained in it is mostly ferrous iron or ferric iron. Where the iron is mostly ferrous iron, slaked lime is usually used to neutralize the water to pH 7 or 8, so that the ferrous iron settles. However, where the iron is ferric iron, ph 3 or 4 is sufficient to settle the iron and, therefore, fine powder of limestone can be used to neutralize the water to that value. Then, since limestone is about a third or fourth the price of slaked lime, changing the iron to ferric iron is an expedient step for an economical treatment of polluted water where the iron contained in it is ferrous iron. As described before, bacteria called "Thiobacillus ferrooxidans" oxidize ferrous iron to ferric iron, and so it has been in use for an economical treatment of polluted water containing a large amount of ferrous iron. The bacteria are loaded on a wool-like, netlike, or scrub-brush-like bacteria-supporting medium, and a number of such media are introduced into an oxidizing tank.
The conventional supporting medium, however, has a drawback that its surface area on which to load the bacteria is relatively small. Another drawback is that the solids in waste water are apt to get into the medium and stay there. A third drawback is that relatively large spaces, into which no portions of the media project, may exist between the media and, therefore, the polluted water may not be treated sufficiently. A fourth drawback contrasts with the third one. That is, if such flow passages are not formed between the media, then on the contrary the media may come into contact with each other at their surfaces. The fourth drawback also results in insufficient treatment of the water.